Dielectric materials are used extensively in a variety of electrical devices. Particularly important is the use of dielectric materials in microwave devices. The requirements for such materials are quite stringent and considerable work has been done in recent years to optimize the dielectric properties of material for application to microwave devices. For the purpose of this application, microwave devices involve devices processing electrical signals in the frequency range above 0.5 GHz; generally up to 200 GHz.
Dielectric materials have a variety of applications in microwave devices. In general, microwave devices using dielectric materials are smaller in size than microwave devices without dielectric materials. Also, such devices with dielectric material exhibit superior device characteristics. For example, microwave filters can be made with superior bandpass characteristics and sharper cutoff characteristics. Larger fractions of microwave frequency allocations are usable for high quality transmissions.
A variety of microwave devices may be made using dielectric material. Typical devices are dielectric resonator filters, microwave strip line circuits and various types of oscillators as well as bandpass filters, etc.
The material requirements for microwave devices are extensive and difficult to meet. First of all, a moderately high dielectric constant is required so as to ensure sufficient interaction between microwave radiation and material. In addition, low loss at the appropriate microwave frequency is highly desirable both to reduce microwave losses and to ensure sharp cutoff characteristics. Low loss or high Q requirements are usually difficult to obtain at microwave frequencies and generally limit the choice of materials useful in microwave devices. Besides moderate dielectric constant and low loss, it is also highly desirable that the material be highly temperature stable (near constant dielectric constant as a function of temperature) so that the microwave properties of the devices are not affected by temperature. In a recently issued patent (U.S. Pat. No. 3,938,064 issued to H. M. O'Bryan, Jr. et al on Feb. 10, 1976) a dielectric material is described which exhibits modest dielectric constant suitable for microwave devices together with high Q and low temperature variation of dielectric constant. Such a material has been used extensively in the design of various microwave devices with considerable success.
It is highly desirable to increase the long-term stability of such dielectric material including the dielectric stability (constant dielectric constant and Q) and mechanical stability (freedom from cracks). This is particularly true where large pieces of dielectric material are required in the microwave device as, for example, in the lower part of the microwave frequency range (0.5 to 6 GHz or more, particularly 0.5 to 1.0 GHz). Also, improved Q factor of such dielectric material is highly desirable because this permits greater frequency stability and more desirable device characteristics.
The O'Bryan, Jr. et al reference cited above discloses a procedure for reoxidizing ceramic material in the temperature range from 900 to 1400 degrees C. in oxygen-enriched atmosphere for 10-100 hours.